The key difference between photochromic and thermochromic is that photochromic materials darken upon exposure to UV radiation, whereas thermochromic materials change their colour upon changes in temperature.
The terms photochromic and thermochromic are mainly used in the context of lenses where the colours change due to changes in some factors like frequency of light and surrounding heat. These are very important terms in analytical chemistry.
CONTENTS
1. Overview and Key Difference
2. What is Photochromic
3. What is Thermochromic
4. Side by Side Comparison – Photochromic vs Thermochromic in Tabular Form
5. Summary
What is Photochromic?
The term photochromic refers to material that can change their colour upon a change in the frequency of incident light. The most common use of this term is as “photochromic lenses”. These are also known as transition lenses. They are optical lenses, and they darken upon exposure to high-frequency light beams such as UV radiation. Therefore, this light beam is named “activating light”. In the absence of this activating light beam, the lenses go back to their clear state.
Figure 01: A photochromic Lens after Exposure to UV light (A part of the lens is covered with a paper and appears in a clear state)
The material from which the photochromic glasses are made can differ; examples include glass, polycarbonate material and plastics. Furthermore, the darkening process of the lenses upon exposure to light occurs faster than the rate of clearing on the absence of the light source. Principally, photochromic lenses are used in eyeglasses; they are dark in bright sunlight and clear in the ambient light state.
When considering the mechanism of colour change in photochromic glasses, we can observe that these glasses get this ability via embedded microcrystalline silver halide in the glass substrate. In plastic photochromic glasses, there are organic photochromic molecules that help to achieve the reversible darkening effect.
What is Thermochromic?
The term thermochromic refers to material that can change the colour upon the change in the surrounding temperature. A mood ring is a good example of this type of material. It is a ring that changes colors based on the temperature of the finger of the wearer.
Figure 02: A Mood Ring
However, there are some other practical uses of thermochromic material as well; e.g. production of baby bottles which can change colour according to the temperature of the liquid inside. Here, the colour indicates when the drink is cool enough to drink. The following video shows the colour change in a thermochromic mug.
There are both organic and inorganic materials we can use for the production of this kind of material. Under the category of organic thermochromic materials, there are two approaches as liquid crystals and leuco dyes. Liquid crystals are used in precision applications, but their colour ranges are limited. Leuco dyes, on the other hand, are less accurate but can be used with a wide range of colours. Under the category of inorganic materials, we can say almost all inorganic compounds are thermochromic to some extent.
What is the Difference Between Photochromic and Thermochromic?
The key difference between photochromic and thermochromic is that photochromic material darkens upon exposure to UV radiation, whereas thermochromic material changes their colour upon changes in temperature. Moreover, photochromic materials are mainly made of glass, polycarbonate material and plastics while thermochromic materials can be either organic compounds or inorganic compounds.
Below is a summary of the difference between photochromic and thermochromic material.
Summary – Photochromic vs Thermochromic
The terms photochromic and thermochromic are mainly used in the context of lenses where the colours change upon changing some factors such frequency of light and surrounding heat. The key difference between photochromic and thermochromic is that photochromic material darkens upon exposure to UV radiation, whereas thermochromic material changes their colour upon changes in temperature.
Reference:
1.“Thermochromic Material.” Thermochromic Material – an Overview | ScienceDirect Topics, Available here.
Image Courtesy:
1. “PhotochromicLens” By User: Vista Lowcost – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Moodring2” By Alkivar – Self-photographed (Public Domain) via Commons Wikimedia
3. “Thermochromic mug” By The wub (CC BY-SA 4.0) via Commons Wikimedia